The airflow through a gas turbine engine is managed by a series of vanes and bleed valves. The vanes typically comprise variable stator vanes (VSVs) and variable inlet guide valves (VIGVs), both of which can be rotated about their longitudinal axes to control the airflow through the engine. This rotation is typically controlled by remote actuators operating mechanical linkages which in turn rotate the vanes.
Accurate control of the vane rotational angle is required to optimise the airflow through the engine and to ensure efficient running of the engine. For example, if VSVs are excessively open the engine may surge, whereas if they are not sufficiently open the engine will not run efficiently and the specific fuel consumption will be greater than desired.
Electrical position sensors could be fitted directly onto the variable vanes for use in accurately controlling their rotation. However, such an approach can be compromised by the harsh operating conditions within the engine. Therefore, a variable vane control system as shown in FIG. 1 may be used. The system 125 has a mechanical linkage 127, typically including a unison ring, operable to rotate a circumferential row of variable vanes. The mechanical linkage 127 is connected to e.g. piston-based actuator(s) 129 such that the actuator(s) 129 can control the angle of rotation of the vanes. In addition, a linear variable differential transducer (LVDT) position sensor 131 may be attached to the, or each, actuator 129 to detect the actuation position of the actuator 129. A controller 133 controls the actuator(s) 129 and thereby controls the angle of rotation of the vanes, the controller 133 receiving the detected actuation position(s) which it correlates with the vane rotation angle.
To improve the accuracy of the system, a time-consuming and expensive rigging procedure can be implemented. The procedure includes shimming the actuator during production assurance testing to calibrate the LVDT reading to the physical actuator position and carrying out an LVDT calibration check. Additional rigging features can be incorporated into the actuator so that a specialist rigging tool can be used to lock the actuator piston at specific positions relative to the actuator body. Similarly, when such an actuator is installed on an overhauled engine, the actuator is re-locked at the rigging position and the vanes locked into an equivalent position before the actuator is connected to the variable vanes via a linkage of adjustable length.
Despite this time-consuming rigging procedure, however, there still remain many factors that are difficult to accurately account for when the detected actuator position is correlated to the vane rotation angle. These factors include: engine electronic controller (EEC) and LVDT electrical errors; calibration and rigging allowances during assembly of the engine; and backlash effects within the mechanical linkage. In an attempt to allow for these factors, actuators are controlled with a safety margin to ensure that surge does not occur. However the larger this margin is, the lower the engines running efficiency will be.